Everything about Creatine Kinase totally explained
Creatine kinase (CK), also known as
creatine phosphokinase (CPK) or
phosphocreatine kinase, is an
enzyme expressed by various tissue types. It catalyses the conversion of
creatine to
phosphocreatine, consuming
adenosine triphosphate (ATP) and generating
adenosine diphosphate (ADP).
In tissues that consume ATP rapidly, especially
skeletal muscle, but also
brain and
smooth muscle, phosphocreatine serves as an energy reservoir for the rapid regeneration of ATP. Thus creatine kinase is an important enzyme in such tissues.
Clinically, creatine kinase is assayed in blood tests as a marker of
myocardial infarction (heart attack),
rhabdomyolysis (severe muscle breakdown),
muscular dystrophy and in
acute renal failure.
Types
In most of the
cell, the CK enzyme consists of two subunits, which can be either
B (brain type) or
M (muscle type). There are, therefore, three different
isoenzymes: CK-MM, CK-BB and CK-MB. The genes for these subunits are located on different
chromosomes:
B on 14q32 and
M on 19q13. In addition to those, there are two
mitochondrial creatine kinases, the
ubiquitous and
sarcomeric form.
Isoenzyme patterns differ in tissues. CK-BB occurs mainly in tissues, and its levels do rarely have any significance in bloodstream. Skeletal muscle expresses CK-MM (98%) and low levels of CK-MB (1%). The
myocardium (heart muscle), in contrast, expresses CK-MM at 70% and CK-MB at 25-30%. CK-BB is expressed in all tissues at low levels and has little clinical relevance.
The mitochondrial creatine kinase (CK
m), which produces ATP from ADP by converting creatine phosphate to creatine, is present in the mitochondrial intermembrane space. Apart from the mitochondrial form, there are three forms present in the cytosol—CK
a (in times of acute need, produces ATP in the cytosol at the cost of creatine phosphate), CK
c (maintains critical concentration of creatine and creatine phosphate in the cytosol by coupling their phosphorylation and dephosphorylation respectively with ATP and ADP) and CK
g (which couples direct phosphorylation of creatine to the glycolytic pathway (see
glycolysis)).
Laboratory testing
CK is often determined routinely in emergency patients. In addition, it's determined specifically in patients with
chest pain and
acute renal failure is suspected. Normal values are usually between 25 and 200 U/
L. This test isn't specific for the
type of CK that's elevated.
Elevation of CK is an indication of damage to
muscle. It is therefore indicative of
injury,
rhabdomyolysis,
myocardial infarction,
muscular dystrophy,
myositis,
myocarditis,
malignant hyperthermia and
neuroleptic malignant syndrome. It is also seen in
McLeod syndrome and
hypothyroidism. The use of
statin medications, which are commonly used to decrease serum cholesterol levels, may be associated with elevation of the CPK level in about 1% of the patients taking these medications, and with actual muscle damage in a much smaller proportion.
Lowered CK can be an indication of
alcoholic liver disease and
rheumatoid arthritis.
Isoenzyme determination has been used extensively as an indication for myocardial damage in heart attacks.
Troponin measurement has largely replaced this in many hospitals, although some centers still rely on CK-MB.
Further Information
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